This invention concerns new compounds and the use of such compounds as electrically conducting and electronically active materials. In particular it relates to the use of sidechain random, or more specifically statistical copolymer systems comprising covalently linked polymer chains bearing electrically active organic substitution.
It is recognised that a variety of organic compounds can be made to conduct electricity. The mechanism of electrical conduction may be either ionic conduction or electronic conduction. Ionic conduction is most important in blends of polymer materials with ionic compounds which may be regarded as solutes in the polymer. Movement of the ions under an applied electric field results in a flow of electric current. Typical materials in which ionic conductivity is important include blends of lithium salts in polymers such as poly(ethylene oxide).
Electronic conduction occurs in organic compounds even in the absence of addition of ionic salts. Various classes of organic compounds show this class of conduction, including conjugated polymers such as poly(acetylene), poly(phenylene vinylene) and poly(thiophene). This class of conduction is also found in molecular solids of low molecular mass such as N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-ditolylbenzidine and aluminium tris 8-hydroxyquinolinate. Polymers which do not possess a conjugated main chain structure can also show electronic conductivity. Such polymers include poly(vinyl carbazole) as a known example.
Electronic conduction in these organic compounds relies on the insertion of electrical charge into the materials. Such electrical charge may be introduced in a variety of known ways, including doping with an oxidising or reducing agent, by chemical doping with a charge transfer reagent, or by direct insertion of positive or negative electric charge from a conducting electrode. Known examples of such introduction of charge include the chemical doping of poly(acetylene) with the oxidising agent iodine, doping of poly(vinyl carbazole) with the charge transfer reagent trinitrofluorenone, and the injection of charge into evaporated thin films of 3-biphenyl-5-(4-t-butylphenyl)oxadiazole from a low work function electrode such as a magnesium metal electrode by application of a negative potential to the electrode. This injection of charge may also be regarded as an electrochemical reduction of the conducting material, or as electrochemical doping of the material. Other electronically conducting organic compounds may be subjected to charge injection by a positive potential, which is commonly applied via an electrode composed of a high work function metal such as gold. Such charge injection is commonly described as injection of positive charges denoted holes, and this description is understood to be equivalent to the extraction of electrons from the conducting material. A further route to generation of charge in organic solids is the ionisation of molecules of the solid under the influence of an electric field or of incident light, or both. In this case, an electron is removed from a molecule of the material, and captured by another molecule, or by a different part of the molecule. In this way, a pair of separated positive and negative charges is produced, movement of either or both of which may contribute to conduction in the material.
Conduction within electronically conducting organic solids involves, in practical materials, the transfer of electrical charge from one molecule to another. Such transfer of charge is described by a charge hopping or charge tunnelling mechanism which may allow an electron to overcome the energetic barrier between different molecules or molecular sub-units. In systems such as conjugated polymers including poly(acetylene) and poly(phenylenevinylene) charge can also flow along the conjugated chain by movement of charged discontinuities in the regular bonding sequence of the polymer. In such cases, charge will normally be transferred through the bulk sample by a large number of individual molecules, and hopping or tunnelling remains an important mechanism.
The use of organic electrically conducting materials as semiconductors in the fabrication of electronic devices has been explored by many investigators. Field effect transistors have been fabricated from organic compounds such as poly(acetylene), pentacene, and poly(phenylenevinylene). Light emitting diodes have been demonstrated using a wide range of organic and molecular solids, including N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-ditolylbenzidine, aluminium tris 8-hydroxyquinolinate, 3-biphenyl-5-(4-t-butylphenyl)oxadiazole and poly(phenylene vinylene). Preferably, such light emitting diodes are fabricated using at least two organic semiconductors which transport charge respectively by transport of holes and electrons. Such multilayer diodes tend to attain higher efficiency than single layer devices, by trapping the charge carriers in the device until carrier recombination and light emission can take place. Photovoltaic devices have been fabricated using compounds such as copper phthalocyanine and perylene bisbenzimidazole. Photoconductive materials have been prepared and are recommended for use as sensitive layers in electrophotography, photocopying and printing applications. Such photoconductive compositions include compositions based on poly(vinylcarbazole) doped with trinitrofluorenone.
Conducting polymers may be used as components of optical storage and switching equipment by using the photorefractive effect. Photorefractive layers combine the capacity for photogeneration of charge, transport of charge carriers by diffusion or under an applied field and a linear electro-optic coefficient. Such properties may be obtained by addition of selected dopants to a conducting polymer. Suitable dopants for inducing the capability for photogeneration of charge include C60 fullerene. Suitable dopants to provide a linear electro-optic coefficient include dimethylamino nitrostilbene.
Many shortcomings have been identified in organic conducting materials described in the prior art. Among these shortcomings, chemical stability is an important parameter. Poly(acetylene), which shows a high electrical conductivity in the doped state, is converted to a non-conducting product on exposure to air, and must be handled and used in an inert atmosphere. Poly(phenylenevinylene) is believed to undergo oxidation of the conjugated double bonds in the main chain, yielding non-luminescent oxidation products. Further degradation mechanisms have been identified or proposed for poly(phenylenevinylene) when it is incorporated in devices, including sensitivity to ambient ultraviolet radiation. Molecular solids such as N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-ditolylbenzidine may undergo crystallisation, changes in morphology, or melting in operating devices. These effects may cause premature failure or loss in efficiency of the device.
A further important shortcoming which is common in many organic conducting materials rests in the difficulty and high cost of processing the materials. Both poly(acetylene) and poly(phenylenevinylene) are insoluble and infusible materials which are prepared for use in devices by use of a precursor polymer route. Typically a soluble precursor polymer is first synthesised and deposited onto a prepared substrate. A combination of heat and vacuum is then used to chemically convert the precursor polymer into the target product, usually with elimination of smaller volatile molecules of one or more by-products. Processing of the target polymer to a dense film in bulk quantities requires critical control of this step which moreover entails the use of costly and time consuming vacuum processing stages. The final polymer is difficult to further process, and steps such as patterning and lithography are difficult to accomplish. Many attempts have been made to provide solution processable organic conductors. Substitution of alkyl, alkoxy and other flexible organic radical onto the polymers is understood to improve their solubility and processibility. Such substitution, however, may also commonly reduce the mobility of charges in the system, making the product less desirable for device preparation. Said substitution may also change the orbital energies of the system, altering the potential required for charge injection into the material.
Low molecular mass conducting organic materials must be deposited in thin uniform films for use in devices. Such films are commonly deposited by vacuum deposition onto the substrate from an electrically heated boat. This process is relatively time consuming and requires the use of costly high vacuum handling equipment. The time required for use of such equipment is relatively long due to the need for evacuation and outgassing of the materials and equipment at different stages in the process. Therefore this route does not provide means for low cost fabrication of organic semiconductor devices.
One way in which it is known to ease the problems associated with the processing of the materials is to provide single layer devices. This may be done in a number of ways. The use of certain types of polymeric active substances means that the use of more than one layer may not be necessary. It is known to use so-called polymer blends i.e. a mixture of polymers wherein the different functions i.e. electron transporting, hole transporting and in some cases light emitting functions are provided by different polymers. These blends tend to be inefficient for a number of reasonsxe2x80x94one of these being they tend to phase separate. The efficiency of a semi-conductor device may be measured or assessed in a number of waysxe2x80x94these methods include measurement of: quantum efficiency which is the number of photons out of the system per electron hole in and power efficiency which is the luminance per Watt.
Other polymer systems have been developed, for example it is known to incorporate both electron-transporting and hole-transporting functions onto a single polymer backbonexe2x80x94for example see Macromol. Chem. Phys., 199, 869-880, 1998 where ABC triblock copolymers for LEDS are described. It is also known to incorporate hole transporting groups or electron transporting groups with chromophores (i.e. light emitters) onto polymer backbones in order to produce statistical copolymers, for example see Bisberg et al, Macromolecules, 1995, 28, 386-89 and Cacialli et al, Synthetic Metals 75, 1995 161-68.
Despite the numerous efforts made up until now there is a continued need for materials suitable for use in semi-conductor devices which have at least one of the following properties:
desirable charge transport characteristics, in particular an advantageous combination of electronic work function which is one factor determining the electric potential required to inject charge in to the polymer from a metallic or semiconducting electrode, charge carrier mobility, the ability to afford control over emission wavelength and bandwidth, ease of synthesis from readily available and inexpensive starting materials, solubility, film forming ability and high physical and chemical stability of deposited films of the polymer in storage and in operating devices, greater efficiency.
It has now been unexpectedly found that processable conducting organic polymers may be prepared by the random copolymerisation of various organic derivatives. Such statistical copolymers yield glass forming polymers which contain no additional functional groups which might compromise stability. It is further surprisingly found that the charge mobility in such polymers is high and organic semiconductor devices fabricated from them provide excellent performance. Test devices show no sign of crystallisation of the statistical copolymer. Furthermore the polymers unexpectedly show excellent solubility in common solvents and may be processed into uniform films suitable for device fabrication simply by spin coating from solution. Said polymers therefore satisfy the requirements for fabrication of organic semiconductor devices in large areas by inexpensive and rapid processing methods. According to this invention there are provided statistical copolymers of general Formula I: 
wherein m and j are the average number of repeat units of A and B such that:
m=0.1-0.9,
j=1xe2x88x92m,
Q=10-50000;
A and B are independently selected from hole transporting groups and electron transporting groups and are statistically distributed along the polymer chain;
X and Z are independently selected from H, CN, F, Cl, Br, CO2CH3.
Preferably the electron transporting and hole transporting groups are directly attached to the polymer backbone such that there is a direct bond between an aromatic part of the electron transporting group and the polymer backbone and a direct bond between the hole transporting group and the polymer backbone.
Preferably the direct bond is such that it is formed from a monomer bearing a vinyl group attached directly to the charge-carrying group.
Preferably the hole transporting group is given by the following general Formula II: 
Where Ar1 is connected to the polymer backbone and is selected from 1,2-phenylene, 1,3-phenylene, 1,4-phenylene, 4,4xe2x80x2-biphenylene, 1,4-naphthylene and 2,6-naphthylene, and B and C are independently selected from groups of structure Ar2, where Ar2 is selected from phenyl, biphenyl, 1-naphthyl and 2-naphthyl, and Ar2 may be optionally substituted with one or two groups independently selected from OR, NRRxe2x80x2, and N(Ar3)2, where R and Rxe2x80x2 denote C1 to C6 alkyl groups and Ar3 is selected from phenyl, biphenyl, 1-naphthyl and 2-naphthyl and Ar3 may be optionally substituted with one or two groups independently selected from OR, N(Ar3)2, NRRxe2x80x2.
Preferred groups of Formula II include:
N,N-diphenyl-4-aminophenyl
2-(N,N-diphenyl-6-amino)naphthyl
N,N-bis(4dimethylaminophenyl)-4-amino phenyl
N-phenyl N-4-methoxyphenyl 4-aminophenyl
N-phenyl N-4-dimethylaminophenyl 4-aminophenyl
N-4-methylphenyl N-4-dimethylaminophenyl 4-aminophenyl
N-phenyl N-4-diphenylaminophenyl 4-aminophenyl
N,N-bis-4-diphenylaminophenyl 4-aminophenyl
N-phenyl, N(4,4xe2x80x2-Nxe2x80x2,Nxe2x80x2-diphenylamino-biphenylyl) 4-aminophenyl
N-3-methylphenyl N-(4,4xe2x80x2-Nxe2x80x2-phenyl-Nxe2x80x2-3-methylphenyl aminobiphenylyl) 4-aminophenyl
N- 1-naphthyl, N-(4,4xe2x80x2-Nxe2x80x2-phenyl-Nxe2x80x2-1-naphthyl aminobiphenylyl) 4-aminophenyl
Preferably the electron transporting group is given by the following general Formula III:
Z1xe2x80x94E1xe2x80x94[xe2x80x94Z2xe2x80x94E2xe2x80x94]nxe2x80x94Z3xe2x80x83xe2x80x83Formula III
where Z1 is connected to the polymer backbone and E1 and E2 are chosen from 
where A, B and D are independently chosen in each ring from CH and N, and Q is chosen in each ring from S, O, NRxe2x80x3, CHRxe2x80x3, CRxe2x80x3Rxe2x80x3, CHxe2x95x90CH, Nxe2x95x90N, Nxe2x95x90CH and Nxe2x95x90CRxe2x80x3, wherein Z1 and Z2 are single bonds or a group Ar4 which is selected from 1,2-phenylene, 1,3-phenylene, 1,4-phenylene, 4,4xe2x80x2-biphenylene, 1,4-naphthylene and 2,6-naphthylene, Z3 is a phenyl, naphthyl or biphenyl group optionally substituted in one or two positions with C1 to C6 alkyl, Rxe2x80x3 is chosen from C1 to C6 alkyl groups and aryl groups and n=0,1 or 2.
Preferably the aryl groups for Rxe2x80x3 are phenyl.
Preferred structures for E1 and E2 in Formula III include:
Oxazole
Oxadiazole
Benzoxazole
Benzthiazole
Quinoxaline
Thiadiazole
1,2,4-triazine
Phenylquinoxaline.
Overall preferred structures for Formula III include:
4-(5-phenyl-1,3,4-oxadiazolyl)phenyl
4-(5-(t-butylphenyl)-1,3,4-oxadiazolyl)phenyl
6-(2,3-diphenyl)quinoxalinyl
6-(2,3-dinaphthyl)quinoxalinyl
4-(5-(t-butylphenyl)-1,3,4-oxadiazolyl)biphenylyl
5-(2-benzoxazolyl-1,4-phenyl-2-benzoxazolyl).
Preferably m=0.3-0.7
Preferably Q=30-1000.
In Formula I the hole and/or electron transporting groups may also act as light emitters (also referred to as luminescent materials), and/or a separate light emitter may be present in devices utilising such materials.
The structural and other preferences are expressed on the basis of desirable charge transport characteristics, in particular an advantageous combination of electronic work function which is one factor determining the electric potential required to inject charge in to the polymer from a metallic or semiconducting electrode, charge carrier mobility, ease of synthesis from readily available and inexpensive starting materials, solubility, film forming ability and high physical and chemical stability of deposited films of the polymer in storage and in operating devices.
It is believed that the statistical copolymers which possess the hole transporters and electron transporters bound directly to the polymer backbone such that there is a direct bond between an aromatic part of the electron transporter and the polymer backbone and there is a direct bond between an aromatic part of the electron transporter and the polymer backbone are particularly advantageous due to there being a reduced so-called parasitic mass attached to the charge carrier.
Compounds of Formula I can be prepared by various routes. Vinyl(triphenylamine), may be prepared by reaction of an iodobenzene with a diphenylamine. Either or both of these starting materials may bear appropriate substitution which is incorporated in the product triphenylamine. The reaction may with advantage be performed in the presence of finely divided copper at elevated temperatures in a solvent such as dibutyl ether, dichlorobenzene etc, according to procedures known in the art and described, for example, by Grimley et al (Org Magn Reson, 15, 296, (1981)), or by Gauthier et al (Synthesis, 4, 383, (1987)). Vinyl substitution on the triphenylamine may be achieved either by use of a vinyl substituted iodobenzene or vinyl substituted biphenyl in the above reaction, or by substitution of a vacant site on the triphenylamine, or by transformation of another functional group.
An advantageous approach to the synthesis of vinyltriphenylamine monomers uses the coupling of 4-iodo bromobenzene with an optionally substituted diphenylamine in the presence of finely divided copper. The resulting bromo triphenylamine is converted to a Grignard derivative by treatment with magnesium metal in tetrahydofuran, and reacted with vinyl bromide to yield the desired product. Other approaches to the synthesis of vinyltriphenylamines will be evident to those skilled in the art, and may be used with advantage according to the particular nature and pattern of substitution which is desired.
Polymerisation of the various monomers to form the copolymer of structure 1 may be accomplished by known means of ionic or free radical polymerisation and may be carried out via thermal or photochemical initiation.
A further aspect of this invention includes a statistical copolymer obtainable by the polymerisation of hole transporting groups of general Formula II and electron transporting groups of general Formula III such that the hole transporting groups and electron transporting groups are statistically distributed along the polymer chain and the degree of polymerisation of the statistical copolymer is 10-50000.
According to a further aspect of this invention an organic semiconductor device comprises a substrate bearing an organic layer sandwiched between electrode structures wherein the organic layer comprises a statistical copolymer given by general Formula I or any of the preferred embodiments of general Formula I.
It may also be the case that the organic layer further comprises another or a plurality of other organic semiconductor materials, including Hole Transport and/or Electron Transport materials.
The organic layer may further comprise a light emitter which may also be referred to as a luminescent material.
Preferably the light emitter is given by any of the following:
A luminescent dye of the coumarin type with a quantum efficiency of photoluminescence of 0.6 or greater
A boron difluoride/pyromethene dye of the general-type described by L R Morgan and J H Boyer in U.S. Pat. No. 5,446,157
A luminescent condensed aromatic hydrocarbon such as coronene, rubrene, diphenyl anthracene, decacyclene, fluorene, perylene etc., and luminescent derivatives of such compounds including esters and imides of perylene, cyanated fluorene derivatives etc.
A luminescent chelate of a metal including europium, samarium, terbium, ruthenium chelates.
Preferably the organic semiconductor device is an organic light emitting diode device.
Preferably at least one of the electrodes is transparent to light of the emission wavelength of the organic layer. The other electrode may be a metal, for example Sm, Mg, U, Ag, Ca, Al or an alloy of metals, for example MgAg, LiAl or a double metal layer, for example Li and Al or Indium Tin Oxide (ITO). One or both electrodes may consist of organic conducting layers.